1. Field of the Invention
The invention relates to an improved fuel injector for use in a common rail fuel injection system.
2. Description of the Prior Art
In injectors, especially those for common rail injection systems of Diesel engines, the opening of the nozzle needle required for the injection is achieved by means of a servo valve. An actuator, such as a magnet valve or piezoelectric element, opens a valve and lowers the pressure in a control chamber via a so-called outlet throttle. The control chamber is defined by the control piston, which acts on the nozzle needle. The lowered pressure in the control chamber on the effective surface area of the control piston alters the force equilibrium at the control piston and sets the control piston into motion, if the pressure falls below a threshold value. If the valve is closed by the actuator, then an increase in pressure occurs in the control chamber, because fuel is supplied to the control chamber via a so-called inlet throttle from an external pressure supply, such as a common rail reservoir. Because of the pressure increase, the control pistons move in the opposite direction. Relative to the location of the control piston, the outlet throttle is typically placed centrally while the inlet throttle is placed outward from it.
In the opened state of the valve, a pressure is established in the control chamber, whose reduction relative to the pressure of the power supply is determined essentially by the current or instantaneous flows through the outlet throttle and inlet throttle. If the valve is open for a long enough time, the control piston reaches a stop, which may be embodied as a mechanical or hydraulic stop. An injector with a mechanical control piston stop (fixed stop) is shown for instance in European Patent Disclosure EP 0 548 916. However, an injector of this kind is not the subject of the present invention.
An injector with a hydraulic control piston stop has become known from European Patent Disclosure EP 0 661 442. This is the prior art that is the point of departure for the present invention. The mode of operation of a hydraulic stop is such that the control piston, as a result of its altered position, additionally throttles the current flow from the inlet throttle to the outlet throttle. This creates an increased pressure upstream of the additional throttle at the narrowest cross section. The narrowest cross section is conventionally called an “N throttle”. The increased pressure acts on a large part of the area of the control piston with a force that acts counter to an opening of the valve piston. The control piston assumes such a position, and thus determines the opening of the N throttle in such a way that the two pressure forces, along with the other forces on the control piston, put the control piston in a position of equilibrium.
The known hydraulic control piston stop of EP 0 661 442 has the disadvantage that periodic motions of the control piston about the position of the hydraulic stop occur, which cause oscillations of the entire system of the hydraulic stop, which can be either self-excited or externally excited. Because of the oscillations of the hydraulic stop, the nozzle needle oscillates as well, and thus the injection rate also fluctuates. The oscillations are expressed as an undulating course of the injection quantity of the injector, as a function of the trigger time of the actuator.
It is true that the attempt has already been made to reduce the oscillation amplitudes by minimizing the control chamber volume. However, recent developments have shown that this provision is inadequate, and it has structural limits, for instance because the relative tolerance in the volume increases upon minimization, which has adverse consequences for the injection quantity tolerances. Moreover, as the system pressure increases, which is favorable for the engine function, the tendency to oscillation also increases.